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volume of acid required in the final titration), less any volume found
the check determination, the glycerine is calculated : i c.c. normal alkaii=
0-03069 gram of glycerine.

With crude soap-lye glycerine, when this contains more, than 2-5% of
organic residue at 160 [see 7), the residue at 160 obtained as in 6 (above)
must also be acetylated in the manner just described ; if the result thus
obtained corresponds with more than 0-5% of glycerine (on the residue
itself), the excess over 0-5% is deducted from the percentage of glycerine
found in the sample itself.

For distilled saponification glycerine and the like, acetylation of the
organic residue is carried out when this exceeds i%, the procedure being
as before and account being taken only of the excess of glycerine over 0-5%.

B. DICHROMATE METHOD.   Reagents required :

(a)  Potassium dichromate (puriss.), powdered, dried at 110-120 and
kept in a well-closed vessel.

(b)  Standard dichromate solution :  7-4564 grams of dichromate (a) are
dissolved in water to i litre.

(c)  Ferrous ammonium sulphate, to be titrated with the dichromate
as follows : 3-7282 grams of the dichromate are dissolved in 50 c.c. of water
and 50 c.c. of dilute sulphuric acid (g).   A convenient excess of ferrous
ammonium sulphate (e.g., 3-4 grams), accurately weighed, is then added
and the excess determined by means of the dichromate solution (b), a drop
of the liquid being removed from time to time and tested with potassium
ferricyanicle.   The amount of dichromate corresponding with i gram of
pure ferrous ammonium sulphate is then calculated (with pure products
I gram of the sulphate = 1-25 gram of the dichromate).

(d)  Silver carbonate, to be prepared afresh for each operation by treating
140 c.c. of 0-5% silver sulphate solution with 4-9 c.c. of normal sodium
carbonate solution, allowing the precipitate to deposit, decanting off the
liquid and washing once by decantation.

(e)  Basic lead acetate, obtained by boiling 10% neutral lead acetate
solution with excess of litharge for an hour and filtering while hot.

(f)  Potassium ferricyanide in 0-1% solution.

(g)  Dilute sulphuric acid, cone, acid being mixed with its own volume
of water.

Procedure. 20 grams of the sample are made up to 250 c.c. with
water in a 250 c.c. flask. Of this solution, 25 c.c. are treated, in a 100 c.c.
flask, with the silver carbonate (d) and, after about 10 minutes, with 5 c.c.
of the lead acetate (e], the liquid being then made up to the mark and 1-5
c.c. of extra water added to compensate for the volume of the precipitate.
The whole is then shaken vigorously and filtered through a dry filter, the
first 10 c.c. of filtrate being discarded and the remainder refiltered if turbid.1
A small portion is tested to ascertain if fresh addition of the lead acetate
gives a further precipitate : if this is the case, the above treatment is repeated
with a fresh volume of 25 c.c. of the original solution but with 6 c.c. of the
lead acetate ; this is, however, seldom required.

* To obtain a clear filtrate, a few c.c. of 10% sodium sulphate solution may be
added before the liquid is made up to volume,


-wter and dried at 100.   The whole is then le ft * ; a_